In the past several years, microdisplays have begun to displace cathode ray tubes (CRT) in various consumer product applications and to be a desirable near-eye display in certain newer product applications. These applications may include video camcorders, digital still cameras, and the emerging area of head-mounted displays. These microdisplays include miniature display panels made from a silicon integrated circuit “backplane” that can be viewed by a user via a lens system or any optical magnifier. Many microdisplays produce full color images, monochrome images, or black and white images by acting as a spatial light modulator on light provided by a separate light source. Spatial light modulator microdisplays may employ liquid crystal materials, such as ferroelectric or nematic liquid crystal materials, or may utilize other technologies such as miniature mechanical mirrors or other suitable light modulation technology. Alternatively, microdisplays may emit their own light by employing miniature light emitting arrays made from emitters such as electroluminescent phosphors or organic light emitting diodes (OLED). In the case of liquid crystal spatial light modulators, they may be transmissive or reflective in nature. In the case of reflective spatial light modulators utilizing liquid crystals, one suitable arrangement is known as reflective LCOS (liquid crystal on silicon). Other arrangements, with liquid-crystal modulators that are frequently transmissive, include active-matrix backplanes made from thin-film transistors (TFT) of either polysilicon or amorphous silicon, or made from single-crystal silicon that has been “lifted off” of a bulk-silicon wafer, as exemplified by the microdisplay products of Kopin Corporation.
The different microdisplay technologies differ significantly in their drive voltage requirements. For example, the electroluminescent (EL) phosphor displays require pixel drive varying over approximately an 80 V range to switch a pixel from fully OFF to fully ON. EL microdisplays have achieved such drive voltages with backplanes fabricated with doubly-diffused MOS (DMOS) high-voltage transistors as pixel drivers. The nematic LCOS displays do not usually require voltages as high, typically needing voltage swings in the range of 9-18 V, or even as low as 5 V. In the case of LCOS using ferroelectric liquid crystals (FLCs), microdisplay products with pixels switching through only 3.3 V are currently in commercial production by the applicant. The 5 V and 3.3 V LCOS microdisplays have been made with backplanes fabricated in standard-logic CMOS processes having ground rules of 0.5 μm and 0.35 μm, respectively, where the standard CMOS logic provides adequate pixel drive voltage.
The different microdisplay technologies also differ in how they produce color. They may generate color in a field sequential fashion or via simultaneous generation of each of the three color fields using pixels with color triads. Field sequential color means displaying color images one color field at a time. For example, a red field may be displayed, followed by a green field, followed by a blue field. If these separate color fields are sequenced at a sufficiently high rate, the human eye/brain will integrate them together into a perceived full color image.
A further issue with microdisplays is generation of gray-scale images. It is advantageous to fabricate microdisplay backplanes as conventional silicon integrated circuits (ICs). Producing gray scale requires each display pixel to be capable of displaying multiple brightness levels. This can be accomplished by driving an analog-responding pixel emitter or modulator with analog circuitry. Silicon fabrication processes specialized for analog circuitry are known, but again typically cost more than baseline digital processes. Further, design of analog circuitry is more difficult and requires greater effort than design of similar digital circuitry. Analog circuitry is susceptible to a variety of noise and offset effects which can produce unwanted image artifacts if not carefully managed. Thus, it is desirable to provide gray-scale through purely digital circuitry.
A number of techniques capable of producing gray scale through digital drive that are suitable for microdisplays are known in the art. For example, fast-responding emitters and modulators such as those found in plasma displays, electroluminescent displays, light-emitting diodes, the Texas Instruments Digital Micromirror Device and other microelectromechanical (MEMS) devices, and ferroelectric liquid crystals (FLCs) can be driven with two-level drive in such a way that variations in the bright/dark duty cycle are used to produce apparent gray scale. In one class of such techniques, the image data is typically separated into “bit planes,” ranging from the most-significant bit (MSB) plane down to the least-significant bit (LSB) plane, and the image data in the bit planes is written onto the display and held for an interval of duration proportional to the significance. Thus, in a very simple exemplary implementation, a pixel displaying an eight-bit monochrome gray scale would be written to eight times during a video frame, and might change state as many times. In fact, such gray scale techniques are known to produce severe visual artifacts, especially with moving pictures. One class of such artifacts is known as dynamic false contouring. Reduction of such artifacts requires complex variations of the simple example given above, with increased data processing, and more pixel state changes. Furthermore, production of a large number of gray shades, such as 256 gray shades usually required for high-quality video images, results in short LSB intervals during which the pixel emitter or modulator must be able to change states. Production of 60 Hz color images from three sequential color fields, each of which fields comprises an image with the abovementioned 256 levels may require switching in intervals as short as 1/(3×60×255) of a second, which is about 22 μs. For some types of modulators, such as ferroelectric liquid crystal modulators, maintaining response times this fast can be difficult, especially in the lower-temperature portions of the ranges most displays are expected to operate over.
The bit-plane family of gray scale techniques can also be used with more slowly responding display materials such as nematic liquid crystals. In this case the pixel has an analog response to the RMS (root-mean-square) value of an underlying two-level electrical drive. In this case, the slow, averaging nature of the liquid crystal material prevents the occurrence of dynamic false contouring, but another class of artifacts occurs instead. Neighboring pixels driven to adjacent gray values may experience very different drive waveforms. For example, in an eight-bit gray-scale scheme, a pixel driven to gray value 128 (binary 10000000) might be driven high for approximately the first half of a video frame and low for the remainder, while another pixel driven to gray value 127 (binary 01111111) might be driven low for approximately the first half of a video frame and high for the remainder. If these two pixels are physically adjacent to each other, as would be the case if they were part of an image with a smoothly varying brightness, a strong lateral electrical field would be produced at the boundary between the two pixels. This lateral or fringing electrical field often produces in nematic liquid crystals a defect called a disclination. Such disclinations have a visual contrast to the adjacent liquid crystal material, often appearing much darker, and, once formed, are slow to disappear even when the electrical drive conditions that produced them are removed. Thus, brightness variations in the images produced on nematic microdisplays driven with bit-plane type digital drive become “decorated” with undesirable dark lines, which can persist momentarily even when the image content is changed.
Many of the above disadvantages of bit-plane type digital gray scale drive can be overcome by alternative two-state drive schemes that reduce the number of drive transitions per video frame. For example, pulse-width modulation (PWM) drive schemes have previously been used, for example as taught in U.S. Pat. Nos. 5,977,940, 6,249,269, 6,329,974, and 6,525,709. In these examples, each pixel has its own driver, which is typically “reset” to a chosen digital value at the beginning of the video field, and are then switched once (and only once) to the other digital value at a time proportionate to the desired gray value. However, the previous implementations referenced above, while utilizing digital pixel drive, have all relied on underlying analog pixel circuitry to perform a comparison between an analog image value, stored on a pixel capacitor, and a global analog ramp voltage, with each pixel having a analog voltage comparator in it. Analog storage of the image value was chosen to reduce achievable pixel size, since a single capacitor can store an 8-bit image value, replacing the function of eight digital memory registers. These analog implementations, while avoiding the image-artifact issues described above with respect to bit-plane type digital gray scale, all suffer from the practical difficulties previously described for analog circuitry.
It is against this background and with a desire to improve on the prior art that the present invention has been developed.